We have previously reported the formation of an intermediate polymer of clathrin (8S) with a sedimentation coefficient of 27S. Deep etch electron microscopy of this polymer revealed it as a closed tetrahedron consisting of four globular domains, each linked to the others by three 33 nm struts. A clathrin assembly protein AP180 that has near identical electrophoretic mobility to clathrin has been purified by a simpler procedure than has been reported. It has been characterized by equilibrium centrifugation as having a molecular weight of 115,000 instead of 180,000 as seen on SDS gels. It is found to react in stoichiometric amounts with clathrin to polymerize clathrin into the smallest size baskets with a sedimentation coefficient of 130S. We have examined the role of the yet another group of clathrin associated proteins (100kDa-110kDa) previously identified, to account for the variability in the size and sedimentation coefficient distribution of the baskets formed when they are added to pure clathrin. Lower ratio of clathrin to associated proteins gives rise to smaller size baskets (150S) whereas higher ratio gives rise to predominantly large size baskets (300S). It has also been concluded that the 150S type baskets are intermediates in the formation of larger size baskets.